The present invention concerns methods and apparatus for measuring visual acuity in preverbal children, along with methods and apparatus for measuring peripheral vision.
Webster""s Dictionary defines visual acuity as the relative ability of the visual organ to resolve detail; keenness of perception. For adults, acuity is tested very easily using an eye chart and the results are given in the form X/X, for example 20/20. However, there is no good way to measure visual acuity in preverbal children. Knowledge of a child""s acuity is important because children learn using their senses and if they cannot see well this will hinder their development. Therefore, it is important to know if the infant""s vision is impaired as early as possible. Tests that measure visual acuity in preverbal children do exist, the most effective of which is the Forced Preferential Looking Test (the Keller Acuity Test). This procedure is discussed in Visual Acuity Testing in Infants: From Laboratory to Clinic by Velma Dobson.
This procedure involves presenting the child with a series of gray cards, each 25.5xc3x9751.0 cm. Each card contains a 12.5xc3x9712.5 cm square patch of black and white grating located to the left or right of a small, 4 mm diameter, central aperture. The spatial frequency varies from card to card, from 0.23 to 38 cycles/cm. During testing, cards are placed face-down on a table beside the tester, arranged in order of increasing spatial frequency. The tester, who does not know whether the grating is located on the left or the right side of each card, watches the infant""s eye and head movements during repeated presentations of each card. Based on the infant""s looking behavior, the tester decides which cards contain gratings that can be resolved by the infant. Cards for which the infant shows consistent looking behavior are judged to contain gratings the infant can resolve. Cards that do not elicit consistent looking behavior are judged to contain gratings that can not be resolved by the infant. Acuity is estimated as the spatial frequency of the finest grating the tester judges the infant can resolve. This test is fairly short, average test duration is 3-5 minutes for normal infants and young children 4 weeks to 36 months of age and approximately 8 minutes for infants with eye disorders.
The acuity card procedure has many limitations. First, the accuracy of acuity measurements can vary to a great degree and retests are often necessary to obtain accurate results. Second, the results of the test may be skewed by the potential bias of the tester. The tester may be familiar with the infant""s history and make assumptions before the test is given which are reflected in the test results. Third, since the acuity card test is conducted at close distances, it is less sensitive to myopic refractive errors than are the 10- and 20-feet picture and letter acuity tests used with older children and adults. Myopia is a condition in which the visual images come to a focus in front of the retina of the eye resulting especially in defective vision of distant objects. Finally, the lack of durability of the cards can contribute to the inaccuracy of the test. This is due the fact that the face of the card should appear uniformly gray when the grating cannot be resolved. Blemishes on the card can produce consistent looking behavior by the patient which could be interpreted by the tester as recognition of the grating. Accordingly, there continues to be a need for improved methods of testing visual acuity in preverbal children.
A first aspect of the present invention is an automated method of measuring visual acuity utilizing an electronic visual display. The method comprises the steps of:
(a) presenting a fixation target for attracting a subject""s attention on said electronic visual display; then
(b) presenting a test screen on said electronic visual display, wherein said test screen comprises at least two separate fields, with one of said fields containing a first test pattern and another of said fields containing a control pattern; wherein said test pattern is configured to stimulate eye movement by said subject when said pattern is recognizable by said subject; then
(c) detecting whether or not eye movement to said test pattern occurs, the presence of eye movement to said test pattern confirming the discernability of said first test pattern by said subject; and then
(d) repeating steps (b) and (c) above with at least one subsequent test pattern, wherein said subsequent test pattern is different from (e.g., more or less difficult to discern) said first test pattern; and then
(e) determining said subject""s visual acuity from the presence or absence of detected eye movement to said first test pattern and said at least one subsequent test pattern.
A second aspect of the invention is an automated method of measuring the quality of peripheral vision utilizing an electronic visual display. The method comprises the steps of:
(a) presenting a fixation target for attracting a subject""s attention on said electronic visual display; then
(b) presenting a peripheral target on a substantially semicircular path, wherein said semicircular path has a first endpoint, a second endpoint, and a center point overlying said fixation target, and wherein said peripheral target is presented at various spaced apart positions along said path, and wherein said peripheral target is configured to stimulate eye movement confirming the recognition of said peripheral target by said subject; then
(c) detecting whether or not eye movement to said peripheral target occurs, the presence of eye movement to said peripheral target confirming the recognition of said peripheral target by said subject; and then
(d) determining said subject""s peripheral vision quality from the point on said semicircular path where the presence of eye movement was detected.
Apparatus for carrying out the foregoing methods are also disclosed herein.